IF YOU cut the maps of various continents along the coastlines and juggle them around like pieces of a jigsaw puzzle, you can almost fit them into one big landmass — the super-continent.
Once you have done that, it is quite natural to conjecture: Were all the continents part of a huge super-continent, which Wegener called Pangaea (all lands)? That was the genesis of the theory of continental drift.
The concept has a history of about 200 years, but it was first proposed consistently in 1912 by Alfred Wegener (1880-1930). Wegener was not a geologist; by education he was an astronomer (with a PhD) and by profession a meteorologist. As it happens now and then, remarkable ideas, concepts, hypotheses and theories are formulated by some very unlikely persons.
Consider Charles Darwin, who read medicine and then thought of becoming a priest. Professionally, he was neither a geologist nor a natural scientist. But he ended up studying natural science all his life. He propounded the epochal theory of natural selection.
There are certain superficial similarities in the concepts proposed by Darwin, on the one hand, and Wegener, on the other. Darwin’s theory challenged the age-old idea of creation of plant and animal life in fixed species for all times, by the theory of biological evolution. Wegener’s theory of continental drift shattered the idea of the permanence of various continents in their existing conditions.
The continents, according to the prevailing belief, were fixed in their positions; they were what they were, so to say, and they were always like that. Lastly, Charles Darwin called his book on evolution The origin of species by means of natural selection, while Wegener called his work The origins of continents and oceans.
It can also be mentioned that a macro-evolutionary change needs a very long time to occur, may be millions of years, and similarly the continents took millions of years (about 200 million years) to drift to their current positions. They are still drifting although the velocity is only of the order of 10cm per year.
One of the significant differences between physical and natural sciences is that a physical theory gives predictions that can be checked and verified frequently in laboratory experiments. Natural science, on the other hand, doesn’t make predictions in this manner.
The data on which it is based can possibly be interpreted in some alternative manner too. Therefore, checking and verification of a natural science theory requires long and patient work and coming together of several other disciplines.
Wegener’s idea had existed for a long time; other people had also thought about it and speculated. But it hadn’t caught fire as it did when Wegener formulated it. Wegener’s formulation was noticed and some other investigators also contributed to it, but since no plausible mechanism of drift was given, it remained buried for many years.
Wegener had proposed that drift was caused by the force of gravitation and Earth’s rotation. Calculations, however, showed that the two forces were not big enough to cause drift. Wegener had hypothesized that thermal convection in the mantle of Earth could also cause drift. But he did not develop this idea in a comprehensive fashion.
Arthur Holmes elaborated upon the convection hypothesis further in 1929, when Wegener’s theory had almost been pushed off the stage. Convection current is caused when a substance is heated, like water in a pan, and its density decreases; consequently the heated substance rises to the top. The denser matter sinks to replace it. Thus, a re-circulating current is generated.
This current “may be strong enough to cause continents to move,” it was suggested. Holmes explained further: “...(T)his thermal convection was like a conveyor belt and that the upwelling pressure could break apart a continent and then force the broken continent (pieces) in opposite directions carried by the convection currents.”
It was also argued that the lines along which the continents were torn apart were more or less like the tear of a newspaper.
When you bring the torn pieces together, they not only fit together, their printed words also wedge together. The implication was that the rock structures of the broken continents, their fossils, fauna and flora were similar along the tear lines.
In due time, when such information was gathered, the proponents would use it to support their thesis, while the antagonists would use the same data to reject it. This is the downside of a theory whose validation is derived only from interpretation of data.
This is explained in the following lines from George Gaylord Simpson’s 1943 paper titled Mammals and the nature of continents: “There are three alternative fundamental hypotheses as to the nature of continents: that they are crustal (of the crust of Earth), segments permanent as entities but variable in position (drift hypothesis), that crustal positions do not vary significantly but continental segments and ocean basins do (transoceanic hypothesis), and that neither crustal positions nor the major distribution of continental and oceanic segments have varied greatly during at least the later stages of Earth history (stable continents hypothesis).”
Of these three hypotheses, he discarded the first two, supporting only the stable continents hypothesis. He used his plant and mammalian data to support his argument. He wrote, “The evidence definitely opposes drifting or transoceanic continents and favours stable continents. Statements of intercontinental fauna resemblances are often misleading and their interpretations have usually been subjective, unreliable, and unscientific.” He also quoted the palaeontologists who agreed with him.
“The fact that almost all palaeontologists say that palaeontological data oppose the various theories of continental drift should perhaps, obviate further discussion of this point and would do so were it not that the adherents of these theories all agree that palaeontological data do support them. It must be almost unique in scientific history for a group of students admittedly without special competence in a given field thus to reject the all but unanimous verdict of those who do have such competence.”
Simpson’s arguments were typical; many others also rejected the theory using similar arguments. The theory was also criticized for the defects in the original idea of piecing together all the pieces of the jigsaw puzzle. Some continental areas are left out in the Bullard’s (maps) fit. Some other areas overlap. Other fits also suffer from similar shortcomings.
For these reasons, the theory would rest almost in oblivion until the 1960s, when different types of evidence became available to bring the drift theory into the limelight.
Tectonic plates
Though there is still no completely undisputed evidence in support of Wegener’s theory of continental drift, there is, by and large, an agreement that the continents do move. This proceeded from Harry Hess’s work in the 1960s.
Harry Hess made a curious observation during World War II. He was a naval officer deputed on a destroyer, which escorted convoys. In order to detect the German submarines, his ship towed a sensitive magnetometer, which would fluctuate due to the effect of the steel hull of a submarine if it were in the vicinity. He noticed that when his ship passed over the mid-Atlantic Ridge, the magnetometer recorded small fluctuations in the magnetic field intensity.
To get to the bottom of this observation, he went to Princeton after the war. His work there culminated in the formulation of the theory of plate tectonics. He became head of the geology department and remained at Princeton University until his death.
A tectonic plate is basically a piece of the Earth’s crust (lithosphere). According to Wikipedia, “The surface of the Earth consists of seven major tectonic plates and many more minor ones. The plates are about 100km thick and consist of two principal types of material: oceanic crust and continental crust. Under both lies a relatively plastic layer of the Earth’s mantle called the asthenosphere, which is in constant motion. This is in turn supported by a solid layer of mantle…. The churning of the asthenosphere carries the plates along in a process known as continental drift, which is explained by the theory of plate tectonics.”
The main features of plate tectonics are (http://www. ucmp.berkeley.edu/geology/tecmech.html):
- The Earth’s surface is covered by a series of crustal plates;
- The ocean floors are continually moving, spreading from the centre, sinking at the edges, and being regenerated;
- Convection currents beneath the plates move the crustal plates in different directions, and;
- The source of heat driving the convection currents is radioactivity deep in the Earth’s mantle.
This is the study of the magnetic properties of the ancient rocks and sediments. It is known that the Earth’s magnetic field reverses from time to time (at intervals of hundreds of thousands years or more). This was confirmed by studies of the sea floor with magnetometers.
The studies revealed the existence of numerous parallel strips of congealed rock, which were formed by the cooling of magma that erupted periodically from the Earth’s core and flowed away from the ridges, on both sides, pushing the older rocks farther away. Adjacent strips had opposite magnetic polarity, which was probably due to the reversal of the Earth’s polarity. Geologists have also found that rocks found in different parts of the Earth with similar ages have the same magnetic characteristics.
Another phenomenon was noticed from progressively older rocks in the same continent, that the magnetic poles appear to wander with time. According to David Pratt (Plate tectonics: A paradigm under threat, http://ourworld.compuserve.com/homepages/DP5/tecto.htm), “Different continents yield different polar wander paths, and from this it has been concluded that the apparent wandering of the magnetic poles is caused by the actual wandering of the continents over the Earth’s surface.”
In conclusion, it may be mentioned that Wegener, when he postulated his theory, was viciously attacked by many not only for his theory but also for his lack of credentials. I. Bernard Cohen remarked in his book, Revolution in science: “The Wegener hypothesis aroused hostility on a number of grounds. First, it went directly counter to the mindset of almost all geologists and geophysicists who had been conditioned from their earliest days to think of the continents as essentially stable, of the Earth as terra firma…. Wegener not only was attacked for his method but was denied the right to discuss geology because he lacked credentials, being a meteorologist rather than a geologist.”
Gaylord Simpson persisted until 1978 in his objections to the continental drift theory. According to Cohen: “... (A)s late as 1978, George Gaylord Simpson repeated his earlier opinion that most of Wegener’s supposed palaeontological and biological evidence was either equivocal or simply wrong; he criticized Wegener for daring to venture into fields with which he had no first hand acquaintance.”
Wegener had a number of supporters also. As the knowledge in geology advanced and new fields of knowledge — for instance, plate tectonics and palaeomagnetism — came into being, the evidence piled up in support of his concept, although not his theory verbatim. The concept of stable and stationary continents has been abandoned and has given way to the drifting ones.
Bernard wrote, “The general shift in earth science from stabilism to mobilism — specifically to ideas of continental drift and plate tectonics — is undoubtedly a revolution....”
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